The present invention relates to a method and a device to increase the capacity, reduce the pressure drop and improve the degree of separation in dry cleansing plants for exhaust gas from aluminum reduction furnaces.
The exhaust gas from aluminum reduction furnaces contains among other things strongly polluting fluorine compounds, substantially as a gas (HF) but also in a form of fluorine containing dust. The dust consists of very small particles of fluoride which evaporate from the smelting bath in the furnace and sublimate in the exhaust gas.
The exhaust gas must be cleansed for fluoride and there are today very strict requirements to the cleansing effect. The so called dry cleansing method is at present pretty universal in this area. This is known technology, and there are several different types. The cleansing technology in these plants are based on the condition that the raw materials for aluminum production, aluminum oxide or alumina, which are powder materials, have the property of dry absorption of HF. The exhaust gas is therefore brought in contact with alumina which can absorb the HF contents. The dust forms fluorides, which must be removed by filtering.
Practically all dry cleansing plants of this type are arranged in such a way that the exhaust gas first comes into a reactor where it is brought into more or less intensive contact with alumina for the adsorption of HF, whereupon the gas passes to a bag filter (textile filter) for separation of particulate material. Most of the fine fluorine containing dust and at least a part of the alumina from the reactor will accompany the exhaust gas into the filter.
The removal of fluorides, both as gas and dust, from the aluminum furnaces, is a loss in the production process. But used alumina from the dry cleansing process, which has absorbed HF from the exhaust gas, and the fluorine containing dust that has been separated out in the bag filter, are led as raw material back to the furnaces. This is how a substantial part of the fluorine loss from the furnaces is recovered. Both the high efficiency and economy in fluorine recovery have made the dry cleansing system universal.
In practice, a greater part of the alumina is used in production. First it is used as an absorbent for HF in the dry cleansing plant, and then it is led back to the furnaces, where particulate fluoride is separated in the bag filter. The fresh alumina shall be referred to as primary alumina, while the fluorine containing alumina from the cleansing plant will be referred to as secondary alumina.
The quantities of exhaust gas from the furnaces in the aluminum industry are very large. The dry cleansing plants are therefore usually divided into sections, where each sections comprises a substantially vertical reactor with a discharge into the bag filter. Adsorption of a HF occurs mainly in the reactor in that the even flow of primary alumina is blended in the exhaust gas at the input to the reactor. Alumina is a powder with a grain size substantially in the area of 40 to 150 xcexcm. Such powder easily is spread like a cloud of dust in the exhaust gas, and provides good contact for the adsorption of HF, but the powder is also coarse enough to be easily separated out of the stream of gas by a dynamic effect, for example by deflection of the gas stream (cyclone effect). In most embodiments the mixture of exhaust gas and alumina is led straight into the bag filter, where a part of the alumina will be separated and fall down in the bottom of the filter as a result of dynamic forces, while a part will follow the gas stream further to the filter bags and be separated there. The fine fluorine containing dust in the exhaust gas has a particle size in the area of 0.1 to 1.0 xcexcm. It is hardly affected by the dynamic forces, but substantially follows the gas stream to the filter bags.
The bag filters in these plants are for the most part the type with rows of stretch out textile bags, where the dust settles on the outside of the bag cloth. The bags are cleaned one row at the time in operation with internal pulses of pressurized air. A layer of dust on the bags will then fall off and down into the bottom hopper of the filter. There it is mixed with alumina which has passed through the reactor and has been separated by dynamic forces.
The necessary filter area which filters out the dust and alumina from the exhaust gas, determines the size of such dry cleansing plants. The pressure drop over the dust covered filter surface also constitutes the greater part of the pressure drop through the cleansing plant, and is therefore a determining fact for the plants power requirements.
The pressure drop over the dust covered filter surface is for the most part dependent on the consistency of the dust layer. In this connection the coarse grained alumina provides a porous dust layer with a low pressure drop which provides great throughput of gas and great filter capacity.
The fine fluorine containing dust, on the contrary, will close the spaces between the alumina grains, increase the power drop through the dust layer and reduce the capacity. The fine dust also easily penetrates the filter cloth and gives a certain content of fluorine carrying dust in the cleansed gas.
Most critical for the plant""s cleansing effect for total fluorine is the adsorption of HF in the reactor. The quantity of alumina in contact with the exhaust gas in the reactor is essential for effective contact and absorption. To increase the quantity of alumina in the reactor and to increase separation of HF it is usual to recycle the separated alumina from the bottom hopper of the filter back into the reactor together with primary alumina. Modem requirements for a cleansing effect far above 99%, makes it necessary to recycle much of the alumina through the reactor, therefore the alumina must be recycled many times before it is tapped out of the plant as secondary alumina and transferred to the furnaces. Separated fine particulate fluoride accompanies the recycling alumina. The more alumina recycled, the more fine dust remains in the system. Continuous fine dust forming on the filter bags increases the dust layer there, which increases the pressure drop, which limits the capacity and causes increased dust penetration. These effects set a limit as to how much recycling of alumina you can have in such plants.
The present invention relates to a method to limit the effects of recycling alumina in dry cleansing plants. The method consists of separating out at least a part of the fine dust that accompanies the recycling alumina before it is injected back into the reactor, and to lead the separated fine dust out of the system together with secondary alumina, which is tapped out of the cleansing plant in a steady stream to be led back to the furnaces. Even a partial, but continuous separating or removal of fine dust from alumina which is recycled in the reactor-filter-system causes a substantial reduction in the quantity of fine dust in the system and of the negative effects of the filtering process.
To separate fine dust from a mixture of fine and coarse particles, the inventors have made use of differences in natural drop velocity and flow properties for fine and coarse particles in motion. Several apparatus that make use of these principles to separate and lead away fine dust from the recycled alumina that flows in a dry cleansing plant according to the description, have been constructed and tested with good results.